1. Field of the Invention
The present invention relates to analog-to-digital converters, and in particular, to a structure for adaptive multi-bit delta and sigma-delta modulation.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the Section entitled “Publications” in the Detailed Description of the Preferred Embodiment. Each of these publications is incorporated by reference herein.)
The signal-to-noise (SNR) performance of the source coder varies with the strength of the input signal. In pulse code modulation (PCM), for example, the SNR is proportional to the ratio V/σx where V is the full scale amplitude of the coder and σx is the standard deviation of the input signal. Waveform coders are also expected to have good dynamic range performance, i.e., to have high SNR even for small input strength. It is well-known that log-PCM (e.g., μ-law and A-law PCM) improves the dynamic range of the coder by reducing, but not eliminating, the dependence of SNR on the ratio V/σx. Another method to increase the dynamic range of coders is to use adaptive delta modulation (ADM) and adaptive sigma-delta modulation (ASDM) systems. See, for example [1]-[6] for overviews on data converters and their applications. Still, conventional ADM and ASDM systems tend to be one-bit coders and their performance is therefore limited by this fact.
In previous works [7, 8], two adaptive delta and sigma-delta modulation schemes have been proposed. While these schemes still employ single-bit quantization, they were nevertheless shown to exhibit superior tracking performance, high dynamic range and improved SNR compared to other similar schemes. Of course, one way to further improve their performance would be to increase the number of quantization bits.
Doing so will decrease quantization noise and increase the overall SNR. However, in the process of this modification it is useful to make a distinction between quantization bits inside a main loop of a modulator and quantization bits inside an adapter that is used to adapt the step-size of the modulator. The present invention maintains the quantization within the main loop to one bit, but increases the quantization within the adapter to multiple bits in a manner that results in improved performance.
Specifically, the present invention describes adapters using multi-bit modulation, including a companded differential pulse code modulator, an adaptive sigma-delta modulator, an adaptive delta modulator, and adaptive differential pulse code modulation. The present invention also describes a framework for studying the performance of these adapters by showing how they can modeled in terms of first-order random gain models. Performance measures are derived from these simplified models and simulation results are then used to illustrate a good match between theory and practice.